The invention concerns an apparatus for the processing of plastic melts, and particularly for the extrusion of plastics, said apparatus comprising at least one feed line which by means of a pump is connected to a vessel containing the yet be processed plastic melts, a housing having a first outlet for the molten plastic to be processed as well as a second outlet for the connecting of a return line leading back to the vessel, and at least one valve for the control of the return flow of the liquid plastic to the vessel. Such an apparatus is described in a prior filed application Ser. No. 493,276 now U.S. Pat. No. 3,947,195 of the same applicant.
Devices for the production of plastic granulates, plastic fibers, etc., often contain a polymerization vessel to which a pump is connected which forces the molten liquid plastic into a die head, from which then emerge the plastic strands, fibers, etc. The polymerization process usually requires several hours, during which time the product is constantly stirred. If only one such vessel is used, the plastic is further processed in batches. During the entire preparation or polymerization time not only the vessel must be continually heated, but also all parts of the apparatus between the vessel and the die head, so as to prevent that the plastic remaining from the previously processed batch does not set up in these components, thereby rendering the apparatus unusable. Many plastic materials, however, are temperature sensitive and would be thermically damaged were they to be kept for long time periods inside the hot pipe conduits and bores between the vessel and the die head considerable waste would result. The same is the case when the process has to be interrupted because a filter for the plastic melt arranged ahead of the die head or build inside it has to be cleaned.
In order to lessen the danger of thermic damage to a certain part of each batch, a device of the above mentioned type has already been suggested (Ser. No. 493,276). In it, upstream of the die head, a three-way valve is arranged, its inlet port connected to a bore or pipe guiding the plastic material to be processed to the die head, and its second outlet part connected to a return line. The liquid plastic fed into the inlet to the apparatus may thus either be processed, or be recirculated into the container. While it is possible in this manner to reduce the quantity of possibly thermically damaged plastic material drastically, such solution can nevertheless not prevent thermal damage when during the switch-over of the three-way valve to back-flow of the plastic material to the container, for example during the change-out of a die head, some plastic material is allowed to remain inside the heated pipes and bores between the three-way valve and the die head for relatively long periods of time. It is precisely this very plastic material which upon resumption of processing is forced through the die first.